1. Field of the Invention
The present invention relates to a screw compressor using a motor as a drive source air-cooled by a cooling fan.
2. Description of the Related Art
Conventionally, a screw compressor driven by a motor including a cooling fan for air-cooling mounted coaxially is publicly known. When the output torque of the motor is T (kg·m), the motor rotation speed is n (rpm), and the compressor power (motor output) is P (W), their relationship is represented by the following expression.T=0. 974P/n
When the discharge pressure is constant, for example, since the output torque T is constant, the compressor power is proportional to the motor rotation speed.
On the other hand, in this motor, a loss is generated at a certain ratio to the motor output, and the loss changes into the motor heat generation quantity. Then, when this motor heat generation quantity abnormally increases the coil temperature of the motor, since the coil presents the insulation failure, it is necessary to prevent the insulation failure, and thus, the motor is air-cooled by the cooling fan. When the coil temperature is maintained constant, since the motor heat generation quantity to be removed by the air-cooling is proportional to the compressor power, the motor heat generation quantity increases/decreases proportional to the motor rotation speed if the motor rotation speed changes.
The cooling airflow quantity from the cooling fan is proportional to the square of the rotation speed.
In case of the screw compressor described above, the cooling fan is disposed coaxially with the motor, its rotation speed is always equivalent to the motor rotation speed, and the relationship between the motor heat generation quantity and the heat quantity removed by the cooling fan, namely the fan-removed heat quantity, is shown in FIG. 7 (horizontal axis: motor rotation speed, vertical axis: heat quantity). The motor rotation speed changes within a certain range, the “MIN” on the horizontal axis indicates its minimum value, and the “MAX” indicates its maximum value. Also, as described above, the motor heat generation quantity shown with a solid line changes in proportion to the motor rotation speed. And, if the cooling fan is designed such that the motor heat generation quantity and the fan-removed heat quantity are equal when the motor rotation speed is at the maximum (SAX), the fan-removed heat quantity changes as a long dashed short dashed line indicates with respect to the motor rotation speed, and the fan-removed heat quantity falls short by a quantity represented by I when the motor rotation speed is at the minimum (MIN).
In contrast, if the cooling fan is designed such that the motor heat generation quantity and the fan-removed heat quantity is equal when the motor rotation speed is at the minimum (MIN), the fan-removed heat quantity becomes excessive by a quantity represented by II as a long dashed double short dashed line indicates when the motor rotation speed is at the maximum (MAX), the fan power is used wastefully, and a problem of acting against energy saving occurs.
As other prior art, Japanese Patent Application Publication S63-213436 discloses art where a cooling fan is provided independently to a motor driving a compressor main unit for blowing air to the motor, and the airflow quantity is controlled according to the motor rotation speed, thereby maintaining the motor temperature constant. The motor rotation speed is detected by detecting the frequency of an inverter.
However, the motor temperature does not depend only on the rotation speed of the motor for driving the compressor main unit. The motor temperature changes under the influence from other different factors. Thus, with the constitution of Japanese Patent Application Publication S63-213436, since the rotation speed of the cooling fan is determined based on the rotation speed of the motor however the actual temperature of the motor might be, it is difficult to efficiently cool the motor.